Many people agree the Big 12 is already overaid and there was specific discussion of a clause that guaranteed new Big 12 teams the same current share but that the contract would NOT get a renegotiation with new teams, a la ACC deal which specifically includes a clause to re-open the deal (welcome abord Syracuse and Pitt).
As for the ACC going to 20 schools, for a long time I never believed it either but high ranking people in the B1G and the Big 12 have already floated this number. Many years ago, it was inquired of the NCAA about 24 member conferences, so somoeone was very far sighted.
Strategically, the B1G is less likely to secure the desired ACC teams without bringing aboard a few more. UNC, UVA and GATech seem to be the most wanted schools, but the B1G would have to destroy the ACC to wedge them out. You can probably through Miami into that group, too. They all want some southeastern partners, just as FSU does (unless they get and SEC invite, then it does not matter).